April 2, 2012

Most animals share a set of derived traits not found in other groups of organisms. These traits make animals unique out of every other organism on the planet. One such trait is specialization. In specialization, cells start out as un undetermined or undifferentiated lump and “grow up” into different types of cells, like skin cells, bone cells, blood cells and nerve cells. However, some animals like sponges lack specialized cells. Advanced multicellular animals have specialized tissues that perform unique functions. Jellyfish, for example (though they are not in the strictest sense “fish”) have specialized stinging cells called cnidocytes that release a deadly poison when other organisms get too close. The process of specialization requires a highly-specific set of genes to regulate an animal’s growth and development. Scientists on Earth have discovered that all animals on Earth share this same set of genes, from organisms as simple as tapeworms to incredibly large and complicated organisms like the blue whale.

This development of animals is controlled by the so-called Hox genes. These Hox genes are responsible for assigning a location and identity to our cells in order to build body parts. Hox genes contain within them a DNA sequence known as the homeobox. The homeobox is a DNA sequence found in Hox genes and other genes that code for transcription factors, which is are proteins that bind to specific DNA sequences in order to activate the transcription of DNA into mRNA. The sequence of amino acids encoded by the homeobox is called the homeodomain. Our so-called Hox genes are a shared set of programs that determine how all complex animals develop. These genes are so important to the development of all complex animals that just about any mutation in this set of instructions is all but assured to result in the death of the organism. This means that there are only a few highly specific ways that these genes could be rearranged without killing the organisms during development. One example of researchers messing with Hox genes resulted with antennapedia, a fruit fly with legs on its head where its antennas ought to be. It provides evidence that evolution is a ongoing process and that natural selection can explain the evolution of all life on Earth. If there is a mutation in the hox genes, it would likely cause a deleterious effect (like building a human with feet where their ears ought to be), and that is the reason why organisms seem to be intelligently designed when in reality they are not. Anything less-than-perfect design that comes by natural selection would get weeded out quickly, leaving only the most exquisitely-designed forms still around. Because of this, virtually every animal on Earth shares the same gene sequence responsible for development. Such genes are astoundingly preserved, maintaining the exact same sequence universally across all deuterostomes over half a billion years of evolution.

This is the single greatest piece of evidence that supports the idea that all animal life on Earth is related through evolution by natural selection. Only random mutation can explain why our Hox genes work just like the set found in every other animal on Earth. And only hox genes can explain why humans develop gills and a tail as a pharyngula just as fish do. During our development, however, the hox genes eventually eliminate the gills and tail. Why would an intelligent designer start to create developing humans as though we were a developing fish, only to remove those fish characteristics anyway later on in development? The only reasonable explanation is that the genes that are responsible for creating all vertebrates during development were preserved by natural selection and that these genes are so fundamental to the success of an individual that we’ve relied on the same set of Hox genes, essentially unchanged since the first vertebrates flopped around in the sea.

The Hox genes that all animal life share on Earth reminds me of something poignant that I learned in biology class as a freshman in college. We are connected in an unbroken lineage to the very earliest lifeforms on Earth. All life shares a last universal common ancestor and it is by the virtue of our ancestors’ integrity that we all can be alive today. And I mean not just our human ancestors, either, but our primate ancestors, our mammalian ancestors, and even our chordate ancestors from 350 million years ago. I marvel sometimes at this fact that on a planet that is over four billion years old, I am alive for at best 100 years, roughly 1-forty-millionth of the history of the planet. And that the human race being around for at most two million years has only been around one-two-thousandth of the history of the planet Earth. Any yet my ancestors carried on in an unbroken chain of creation for over four billion years before I came along. It makes me feel a sense of awe in a way that no religion ever has been able to instill upon me. I am part of a much larger and much older biological community almost as old as the planet itself.